Servocontrol systems are well known for use in the actuation and control of reversible motors. Various damping methods are used in conventional control systems to minimize the number of oscillations made by a motor while braking. One method is disclosed in U.S. Pat. No. 3,378,740, issued Apr. 16, 1968 to R. Cruel in which a damping member is connected in parallel with the motor's armature. In one embodiment, the damping member is a resistor; however, continued reduction of the resistor's ohmic value in an effort to further increase the amount of damping achieved results in a limitation of the speed of the motor as the resistance of the damping member approaches the resistance of the motor's armature. In an alternative embodiment of the Cruel device, a transistor is substituted for the resistor to act as the damping member, in which the transistor's collector-to-emitter path is connected in parallel across the motor's armature. This configuration produces forceful braking of the motor, but only in one direction. No braking force is applied once the motor changes direction, resulting in unbalanced control of the motor.
Another servocontrol device is disclosed in U.S. Pat. No. 3,134,036, issued May 19, 1964 to B. Fisher, et al., in which two duo-triode vacuum tubes are used to compare a feedback signal, indicating a motor's position, to a preselected control signal and to alternately actuate one of two relays causing said motor to rotate in a particular direction. In this circuit, which requires multiple power supply or voltage source which must float independently of each other, proportional actuation of the relays is effected, a method less desirable than an instantaneous application or elimination of threshold voltage since the contact arms of said relays could become fixed in a position midway the contacts due to partial energizing of the relay coils. Furthermore, the deadband range is controlled in this configuration by adjustment of the gain of one duo-triode vacuum tube through the use of a potentiometer connected in parallel with the plates of said vacuum tube. This is an imprecise method of establishing deadband limits, since the resistance value to which the potentiometer is set in order to obtain a certain gain would vary according to the particular characteristics of any given vacuum tube.
Other proposed methods for providing precise positioning control of reversible motors include the use of complex integrated circuitry by which power is turned on and off and applied in alternating directions to control the position of the motor. Although such systems are effective, there is a need for an uncomplicated, efficient and inexpensive closed-loop servocontrol system.
It is, accordingly, an object of the present invention to provide an improved servocontrol system.
It is another object of the invention to provide a simple, but effective, servocontrol system operable in conjunction with a reversible DC motor to provide precise rotational position control of the motor in a closed-loop system having a minimum deadband, to reduce undesirable motor oscillation while braking.
It is also an object of the present invention to provide a servocontrol system which facilitates the adjustment of the magnitude of the deadband, employing a single resistor, thus permitting system flexibility under varying load requirements while maintaining a consistently high level of component sensitivity.
Another object of the invention is to provide a remote load positioning device having a single polarity primary power source and utilizing the counter electromotive force (emf) of the motor to provide dynamic braking, thereby requiring no additional energy provided by the primary power source for restraining the motor.
Yet another object of the invention is to provide a control circuit which is operational with either mechanical switching devices for economy and circuit simplicity, or with solid state switching devices for maximum component longevity and for use in size restricted applications.
Still another object of the invention is to provide a control circuit having dual relays, each operational to cause motor rotation in a particular direction, yet causing the motor to brake when both relays are in a normal, deactivated state, thereby eliminating any need for sequential operation of the relays and resulting in a simple, inexpensive control system.